For many neurodegenerative diseases, both effective treatments and underlying causes are unknown. A specific protein, TDP-43, has recently been shown to be abnormally deposited in multiple neurodegenerative diseases, including Amyotropic Lateral Sclerosis (ALS) and Frontotemporal Lobar Dementia (FTLD). TDP-43 (TARDBP) is a conserved, broadly expressed nuclear protein with demonstrated roles in mRNA splicing and stability. By analogy with other neurodegenerative diseases associated with specific protein inclusions or aggregates (e.g., huntingtin in Huntington's, 1-synuclein in Parkinson's, 1-amyloid peptide in Alzheimer's, etc.), there are good reasons to believe that TDP-43 plays a causal role in neurodegenerative pathology. We have expressed human TDP-43 in C. elegans in order to assess the effects of TDP-43 overexpression in an in vivo model. Our experiments show that transgenic worms with neuronal expression of human TDP-43 have an uncoordinated movement phenotype that is indicative of neuronal dysfunction. Thus, our data support a neurotoxic role for TDP-43. The molecular mechanism(s) by which TDP-43 may be neurotoxic are unclear, in part because the full range of TDP-43 function is unknown. TDP-43-positive inclusions occur in FTLD cases caused by loss-of-function mutations in progranulin but the biological connection between TDP-43 and progranulin has not been established. In this project, we will seek to identify the conserved functions of TDP-43 (and progranulin), and the molecular and cellular mechanisms by which TDP-43 causes neurodegeneration. Specifically, we will carefully characterize C. elegans strains with deletions of TDP-43 and progranulin to determine the functions of these genes at the organismal level. We will also generate and characterize a series of transgenic C. elegans strains expressing variants of human TDP-43 designed to elucidate the molecular mechanisms of TDP-43 toxicity. These transgenic strains will also be used in forward genetic screens to identify components of the TDP-43 neurotoxic pathway. The C. elegans studies will be complemented by parallel cell culture studies, which will serve to validate and extend findings made in the C. elegans model system.